Technical Abstract:
Squash vein yellowing virus (SqVYV) was identified in cucurbits in Florida in 2005, shown to be whitefly-transmissible and to induce a previously observed watermelon vine decline and fruit rind necrosis. SqVYV has been isolated from declining watermelons for the past six growing seasons in southwest and west-central Florida where it has caused significant economic impact to watermelon production. Common cucurbit weeds in south Florida were examined as potential reservoirs for SqVYV as only cucurbits have been determined to be hosts for this virus. Over 40% of balsam-apple (Momordica charantia) plants collected from watermelon growing areas with previously reported vine decline were infected with SqVYV. Creeping cucumber (Melothria pendula) was determined to be an experimental host for SqVYV. These results demonstrate that cucurbit weeds can provide reservoirs for SqVYV in Florida. Sequencing of the SqVYV genomic RNA showed one large open reading frame encoding a single polyprotein predicted to be processed into ten mature proteins. Phylogenetic analysis of these proteins supports classification of SqVYV as a novel species within the genus Ipomovirus in the family Potyviridae. The genome organization of SqVYV (which includes a P1b protein but not an HC-Pro protein) is similar to Cucumber vein yellowing virus but different from Sweet potato mild mottle virus (both recognized members of the genus Ipomovirus) indicating that the taxonomy of this genus needs to be re-examined to accommodate the observed discrepancies in genome organization. The virus disease situation in cucurbits recently worsened in Florida with the detection of Cucurbit leaf crumple virus (CuLCrV). In plant distribution of SqVYV and CuLCrV was determined in 80 watermelon plants collected from a commercial field by using nucleic acid hybridization assays. Results indicate that SqVYV and CuLCrV are spatially separated in watermelon plants, a fact that may effect whitefly acquisition and dissemination of these viruses. Progress of SqVYV and CuLCrV infections was monitored in an experimental watermelon field in a separate study. Symptoms of CuLCrV were present soon after planting. Symptoms of SqVYV first appeared 7 weeks after planting and by week 12 the field was fully collapsed from disease. Results of analyses of the association between the two viruses indicate that they are being introduced randomly by whiteflies. Additional field experiments are in progress to verify and extend these findings.